Elevated numbers of blood and tissue eosinophils are present in allergic diseases and experimental evidence suggests that eosinophils play an important pathogenic role in these conditions. Regulation of eosinophil maturation, recruitment, and survival is under the control of a small group of factors, including interleukin-5 (IL-5). Given the probable importance of eosinophils to allergy and other associated disorders, IL-5 has been proposed as a potential molecular target in the treatment of these diseases. IL-5 antagonist therapies in current development include two monoclonal anti-IL-5 antibodies (mepolizumab, reslizumab), a monoclonal antibody directed at the IL-5 receptor (benralizumab), and anti-sense oligonucleotide therapy (TPI ASM8). Anti-IL5 antibody therapy has been the most extensively studied of these agents, and trials have been performed in patients with bronchial asthma, nasal polyposis, atopic dermatitis, eosinophilic esophagitis, hypereosinophilic syndrome, and Churg-Strauss syndrome. In studies of asthmatics, anti-IL-5 showed minimal efficacy in patients with moderate, controlled asthma. In patients with severe, refractory asthma associated with eosinophilia, however, clinical trials have demonstrated significant reductions in asthma exacerbations. Clinical studies in other disorders, particularly eosinophilic esophagitis and hypereosinophilic syndrome, have also shown significant improvements in blood and/or tissue eosinophilia and variable alterations in clinical disease activity. Strategies aimed at the inhibition of IL-5 may hold great promise in the treatment of eosinophilic diseases.